Automated high-speed production lines for printing sheets of labels or the like conventionally use offset or flexographic printing equipment in which four rotating printing plates (for the three basic colors plus black) print four images in exact superimposed relationship onto a pair of transfer rollers from which the colored image is then transferred to a substrate (paper, plastic or the like) conveyed through the printer on an impression cylinder.
Although the resolution of this printing process is limited, it is satisfactory for many practical applications in which production speed is important. This is typically the case for orders of millions of identical items such as labels in which set-up time and variability are secondary considerations.
A problem arises when equipment of this type is to be used for short-run orders (e.g. less than 100,000 units) or orders in which successive images of a run need to be different in some way. For example, in order to overprint successive images with serial numbers, expiration dates or the like, a separate overprint head must be used after the printing process to affix the overprint. This separate head causes difficultly on the production line because it is prone to miss-registration due to unintended movement of the substrate when the substrate is deposited on a conveyor following the printing operation.
Many printing processes also do not lend themselves to short-run production because any alteration of the image requires replacement and realigning of the printing plates requires a costly and time-consuming operation which is usually not economically feasible.